Photovoltaic devices such as solar cells are capable of converting solar radiation into useable electrical energy. The solar cell semiconductor material can have a crystalline structure, e.g., single crystalline or poly-crystalline silicon, or a non-crystalline structure, e.g., amorphous silicon. Energy conversion occurs as the result of what is well known in the solar cell field as the "photovoltaic effect". Two basic steps are involved in the photovoltaic effect. Initially, solar radiation absorbed by a semiconductor body generates electrons and holes. Secondly, the electrons and holes are separated by a built-in electric field in the semiconductor body of the solar cell. This separation of electrons and holes results in the generation of an electrical current. A built-in electric field can be generated in a solar cell by, for example, a Schottky barrier. The electrons generated at the metal (Schottky barrier) semiconductor body junction flow towards the semiconductor body.
Cermets, also known as granular metals, are composite materials consisting of finely dispersed mixtures of immiscible metals and insulators. They can act as Schottky barriers when applied to single crystalline n-type silicon and gallium arsenide as is known, see for example, J. Appl. Phys., Vol. 45, No. 1, January, 1974. However, due to the differences between crystalline and amorphous silicon, see IEEE Transactions On Electronic Devices, Vol. ED-24, No. 4, April, 1977, no conclusions can be extrapolated as to whether or not a cermet which acts like a Schottky barrier to crystalline silicon will also act like a Schottky barrier to amorphous silicon.
Hydrogenated amorphous silicon photovoltaic devices described in U.S. Pat. No. 4,064,521 are capable of converting solar radiation into useable electrical energy. Schottky barrier hydrogenated amorphous silicon solar cells are fabricated by glow discharging silane (SiH.sub.4) to deposit a body of hydrogenated amorphous silicon and thereafter evaporating a film of platinum or another high work function metal to form a Schottky barrier on the deposited body of hydrogenated amorphous silicon. It has been noted that immediately after the evaporation of the Schottky barrier metal films, the resultant Schottky barrier solar cell has inferior diode characteristics. A good Schottky barrier develops only after a certain prescribed annealing procedure, e.g., annealing in forming gas at about 200.degree. C. for about 15 minutes. This treatment apparently results in the formation of some thin interface layer, thus far of unknown nature. Schottky barriers thus formed are susceptible to the presence of electrical shorts and shunts, which can preclude the fabrication of large area solar cells. Attempts to reduce the cost and speed for processing of hydrogenated amorphous silicon solar cells by sputtering the platinum film results in a higher percentage of cells with shorts or shunts. The incidence of shorts and shunts also increases with increasing solar cell area.
Electrical shorts occur when there is a pinhole in the amorphous silicon body and the front and back electrodes are touching. A shunt is the loss of charge in the amorphous silicon body due to imperfect barrier formation or the formation of an ohmic contact by the work function metal rather than a Schottky barrier. Electrical shorts and shunts either greatly reduce or completely eliminate the electricity generated by the amorphous silicon solar cell.
When the amorphous silicon solar cell is fabricated in a PIN configuration, the transparent conductive oxide or transparent metal electrode tends not to adhere well to the p+layer. Poor adherence of the transparent conductive metal or metal oxide to the p+layer results in poor solar cell performance.
Thus, it would be desirable to find a material which can be applied to a hydrogenated amorphous silicon body by either sputtering or co-sputtering, act as a Schottky barrier or adhere well to a p+amorphous silicon layer in a PIN amorphous silicon solar cell and minimize the effect of pinholes in the amorphous silicon film.